<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div class="qq">
    <h2 class="h2">21354</h2>
</div>
<style>
    html{
        width: 100%;
        height: 100%;
    }
    body{
        display: flex;
        width: 100%;
        height: 100%;
        margin: 0;
    }
    .qq{
        height: 100%;
        width:100%;
        background-color: #55a532;
        margin: 0;
    }
    .h2{
        display: flex;
        width: 100%;
        height: 100%;
        justify-content: center;/*水平居中*/
        align-items: center;/*垂直居中*/

    }
</style>
</body>
</html>
